Radiation absorber



Feb. w, 1947. w. P. MASON 2,415,832

RADIATION ABsoRia'En Filed Dec. 31, 1942 3 Sheets-Sheet 1 Z Z tall z Z I 2 tan 2- 23 I J 0 IF,- I J 0 2y RUBBER L no TE CRYSTAL MOSAIC 4 CASTOIP 5 OIL CRYSTAL PICKUP .41. MOSAIC mesa/v4 ran FIG. 4

FIG. 3

INVENTOR Feb. 18, 1947. w, P, MASON 2,415,832

RADIATION ABSORBER Filed Dec. 31, 1942 3 Sheets-Sheet 2 05C. ATTENUATOR ALLA CRYS TAL MOSAIC BACKING RESOIVI TOR SLOTTED PLATE MICROPHONE PIS TON F/GQ RUBBER DIAPHRAGM F CRYSTJL MOSAIC I I I I I I I ACKIIVG RESONATOR T 'I L 1' R R i z g I 2 :Qsmr" E 0, I

m azzzzz'zps FIG. 8

INVENTOR W I? MASON I w I A TTORNEV structures.

1*" Feb. 18.194?

IJNITE J FHCE RADIATION ABSORBER Warren P. Mason, West Orange, N.-J., assignor to Bell Telephone Laboratorlmlncorporated, New York, N. Y, a corporation of New York 7 known as acoustic resistances are used. These may be in the form of slotted plates or screens and, being placed in the liquid medium between the end surface of the resonator and the casing, act as attenuators and reduce theamount of energy reaching the free medium, the sea. water by way of example.

The use of the so-called backing resonators is disclosed and the theory of their operation is fully expounded in my copending application entitled "compressional wave radiators and receivers, Serial No. 413,429, filedOctober 3, 1941.

Submarine projectors and hydrophones working large distances under the surface, of the ocean are subjected to large hydrostatic pressures. For such devices it is dimcult to provide air spaces inside the units, and it is desirable to have the complete inside volume filled with oil to withstand the pressures without resorting to very massive In such units the crystal quarter wave-length metal backing resonators are surrounded by oil'so that the transmission of waves from the end surface of the resonators is much higher than if they were in an air chamber. Under ideal conditions of resonance, perfect elas ticity of the resonator and other factors the amount of energy which might thus be transmitted would approach zero, but under practical conditions this becomes a factor of importance. For various reasons, including that of efficiency it is desired to reduce the amount of energy reaching the casing of the projector or hydrophone.

The amount of energy radiated from metal backing resonators has been investigated both theoretically and experimentally and means for absorbing this energy so that the amount reaching the casing is small. have been devised and are disclosed herein.

The results of this theoretical and experimental investigation show that for a steel quarter wave backing plate, when thebacking plate and the crystal are both surrounded by an absorbing medium, the steel backing plate-will radiate about one per cent as much energy as the crystal unit.

For a lead backing plate, however, about nine per cent as much energy will be radiated from the lead backing as from the crystal surface.

The analysis also shows that if the metal backing plate alone is placed in the liquid and the crystals vibrate in air, a very sharp resonance curve is obtained similar to the characteristics of a magnetostriction projector. This type of characteristic is of use by way oi example wheremore than one projector is required in a given region. By changing the metal backing plate composition, the breadth of the resonance curve can be controlled.

When a lead or other low impedance backing plate is used in oil, it may be desirable to reduce the amount of sound that it can pick up. This can be accomplished by using a spaced grating or a screen which employs the viscosity of castor oil to introduce series and shunt resistances which will form an acoustic pad capable of attenuating sound from or to the metal backing plate.

A feature of the present invention is an acoustic resistance interposed between a crystal backing resonator and a casing wall in a device wherein the said casing is filled with a viscous liquid medium.

Other features will appear hereinafter.

The drawings consist of three sheets having thirteen figures, as follows:

Fig. 1 is an electrical circuit diagram showingthe relationship between the impedances and other equivalent factors of a vibrating crystal;

Fig. 2 is a similar circuit'diagram showing the effect of a backing plate;

Fig. 3 is a diagram, partly a schematic circuit diagram and partly a sectional mechanical arrangement showing the relationship of certain apparatus used in the experimental determination of the relative radiation from the crystal and its backing plate;

Fig. 4 is a sectional view of an arrangement to take advantage of the sharp frequency response obtainable with a projector which radiates only from the backing resonators;

Fig. 5 is another figure similar to Fig. 3 showing the position of a slotted plate used in expertmental apparatus to determine the effect of one form of an acoustic resistance;

Fig. 6 is a circuit'diagram illustrating the fac-' tors considered in the use of a slotted plate asin Fig. 5;

Fig. 7 is a sectional mechanical view of a complete radiator, showing the use of screens as acoustic absorbers Fig. 8 is a diagram showing how the impedance, of an oil layer and the resistance of two screens may be shown by an equivalent circuit, the diagrammatic circuit at each side being for all practical purposes equivalent to each other;

Fig. 9 is a combination end and side view illustrating the relationship of certain factors used in the formulae relating to the movement of a piston in proximity to an immovable wall;

Fig. 10 is a graph showing the relative radiation from a crystal quarter wave driving unit asiaess and a steel quarter wave-length backing plate cent of the total energy is radiated by the steel plotted as a function of frequency; backing plate. For lead the ratio is 9.85 so that Fig. 11 is a set of graphs showing the relative 9.21 per cent of the total energy is radiated by a radiation from a crystal mosaic and a steel backlead backin plate. The output from the steel ing resonator plotted as a function of frequency: ii is down 19.0 decibels while the lead is down only Fig. 12 is a set of graphs showing correspond- 10.1 decibels.

ence between theoretical calculation and experi- Fig. shows a calculation of Equation 5 over mental determination of the radiation from the a wide frequency range for a steel backing plate backing resonator of a crystal mosaic; and of large area. The relative responses show a Fig. 13 is a set of graphs showing the effect of In greater diflerence for low frequencies but a a slotted grill on the radiation from a backing smaller difference for frequencies above the quarresonator, as determined experimentally. ter wave point. When the back resonator is a half wave-length as much energy is radiated from the Theoretical i gggg g jgz gg from a backing plate as from the crystal surface. For g 15 a frequency range of 1.6, centered around the The relative radiation from t e c ys l fr quarter wave point, the amount radiated from the face s omp d to the me l backing plate is steel backing resonator will be at least 15 decibels easily calculated by iemgiloying $1111; crysiigl equigdown from that radiated from the crystal surface. alent circuit shown n g. 1. s is accor ance with explanations heretofore published in igzj g gg zzf gggz zgzfig 'g ggi an article entitled "Measurement of the elastic, electric and piezoelectric constants of Rochelle In order to check the above theoretical consalt," Physical Review, April 15, 1939. In this clusion certain experiments have been made. representation Ct=the static capacity of the The pparatus for the experimental rr n em n crystal, o the transformation factor, Zo' the meis Shown in 1!? 0011518158 Of a Container chanlcal image impedance of the crystal, and 1; divided into two parts and lined with a metal the velocity of propagation in the crystaL In wool. The small passages in the wool introduce centimeter-gram-second units for a 45 Y-cut considerable losses in the vibrations in castor oil tal 3 and form a dissipative lining which absorbs to l l some extent the standing waves in the castor oil.

C =.796- =8.92 10 l.,,; Z3=4.19 10l,,l,; On a supporting plate 4 of the container there 5 is mounted a crystal mosaic 5 and its backing 235x10 centlmeter per second (1) resonator 6, the crystal mosaic 5 being immersed where 1y is the lengt lw the Width and 1% the in the castor oil of the lower chamber and the thickness of the crystal. backing resonator 6 being immersed in the castor To represent t e backing plate e dd t T oil of the upper chamber. During a portion of network of a mechanical transmission line as th x erime t the crystal and its backing shown in 2 h r 0 h mechanical im resonator are located as shown and in another impedance and v the velocity are given by portion these elements are reversed, the crystal 7-; 40 mosaic 5 being immersed in the castor oil of 0= w l\ P 0;' ='JT (2) the upper chamber and the backing resonator 6 being immersed in the castor oil of the lower where p is the density and Y0 the value of Young's chamber,

modulus. For steel and for lead the values are The radiation in the lower chamber from the given in Equation 3: 5 crystal or backing plate is measured by means stee1 =7.s; Y=2 1o"; Z =3.95 10lelr; v-=5.0sx1oof the longitudinally vibrating Rochelle salt crystal I which is connected to an amplifier 8 L d =ll.3;Y=l.57 1o";z=1.aa 1o z.,z; ==1J8X10 ea p X v and meter 9. The crystal mosaic 5 is driven by In addition we terminate both sides of the oscillator m workm equivalence in radiation resistances Ra and radi- 8 through an attenuator ation reactances Xa representing the reaction of The experimental procedure was to v ary the the medium on the radiating surfaces. These are oscillator m over wide frequency range, with taken equal since we are assuming equal areas for the driving crystal 5 in the measuring side and the crystal and backing plate and equal absorbrecord the results shown by the meter 9. The ing media on the two sides. When both the crysmosaic 5 was then turned around so that it 1794 and the backing plate h large was in the upper chamber and the resonator 6 becomes small and the radiation resistance Ra in the lower chamber with face the same equals tance from the measuring crystal 1 and the run Rs=1.5 10 lwlt (4) repeated The relative radiation from the two faces can Fig. 11 shows the results of such a. test for a be determined by solving for the ratio of ii to 14 steel backing plate. From 30 kilocycles to '70 for the network andsquaring, since both ends kilocycles the diflerence varies from 17 decibels are radiating into equal radiation resistances. to 21 ecibels Or n ve of 1 deci ls which This results in th equation agrees well with the figure obtained theoretical- (i R'i'j R) 1+ a+ 4)+ l+ 3) a'i d'i' z ir (5) h 4( i+RR+j R) When the radiating area is large so that Xa can 1y. Above kilocycles the relative responses of be neglected and the crystal and backing plate the two surfaces become more nearly equal as are each a quarter wave-length, this formula 70 predicted by the curve of Fig. 10.

reduces to the simple form A similar run was made with a lead backing plate and within the experimental error the rem (6) spouse from the lead plate was down 10 decibels over that from the crystal surface in agreement For steel this ratio is 78.5 to 1 so that 1.26 per "1 with calculations.

aeraasa Radiation from hacking plate with crystal in air Magnetostrictive radiating units used in submarine detection have a sharp resonance curve and radiate emciently only over a narrow band of frequencies, whereas most Rochelle salt radiators have a very wide response range. It is the purpose of this section to show that a sharply tuned response can be obtained with a crystal resonator if desired by simply reversing the usual construction by putting the crystal in the air and the backing resonator in the medium. One such unit is shown in Fig. 4. This is essentially the construction of the magnetostrictive type of radiator, which accounts for the narrow frequency range obtained with such units. For

some purposes, however, such characteristics are desirable and hence it appears worthwhile to consider what characteristics are obtainable with a sharply tuned Rochelle salt radiator.

The circuit of Fig. 2 is applicable to this case if we short-circuit the left-hand or crystal radiation end. If we combine all the mechanical elements, the resulting: mechanical impedance becomes where Za is the radiation impedance of the medium. A little consideration shows that the principal resonancefrequency of this combination comes very close to the quarter wave-length frequency or the backing resonator and crystal. If we insert the expression for Z1, Z2, Z3, Z4 and let w=(w l-Aw) where can is the frequency of the quarter wavelength, the impedance Zm takes the form that is the values are the same as those for a quarter wave-length crystal except that the compliance is divided by while the effective mass is increased by the factor The Q of a -degree Y-cut Rochelle salt crystal mosaic radiating into a medium is given by When used alone this gives a rather wide transmission region. The maximum Q that can be obtained from a back plate resonator will be Q 1 =1 u( o+ o) B oo C R 4 natal. (13) For steel and lead the values will be Qn=215 for steel; QB=3L5 for lead. Hence either the lead irAw At the mechanical resonant frequency the radiation resistance is given closely by the expression In order to test out these relationships, a well glued crystal mosaic, glued to a steel backing resonator was obtained. The ceramic plate .was silver plated by chemical deposition and then copper plated to build up the layer. The ceramic was then sweated down to the steel plate. The idea of this bond rather than the' usual glued bond was to eliminate some of the gluing loss since it has been found that the ceramic to steel joint is the poorest by present techniques. The crystal mosaic was then glued to the ceramic by the use of a thin layer of Bakelite glue. The crystal mosaic when measured alone had a Q of 300, while the Q of the crystal and steel resonator had a Q of nearly 200. Since the resistances of the two joints are additive in Q determination, this indicates that the resistance put in by the glued joint between the Rochelle salt and steel is only half as much as that introduced by gluing the mosaic together.

The resonant and anti-resonant frequencies of the complete resonator, the resistance at resonance, and the capacitance at a high frequency were measured with the result fa=58,500 cycles; js=60,540 cycles (14) R=13,500 Ohms; C'c=15pp.f This gives the following constants for the equivalent circuit Co=15 p i; C'1=1.06 p i; L1=6.95 henries (15) R1=13,5O0 ohms; Q=

7 In agreement with the calculation of Equation 11, the value of C1 is slightly lower than for a 45- degree Y-cut crystal alone. For the mosaic vibrating as a quarter wave-bar (i. e., with a perfect clamp) the ratios of capacitances in the crystal is 12, and the value of C; will be 1.18 puf. From Equation 11 the effect of the imperfect clamp should be to decrease this capacitance in the ratio Applying this factor the measured capacitance C1 for this case should be 1.06 oi. which is close to the value found.

The mosaic and backing plate were then put in the experimental arrangement of Fig. 3, with the oil removed from the top section. The backing resonator was immersed nearly up to the top while the crystal was placed in air. The constants of the resonator were measured again with the values Co=18 p i; C1=1.06 nah; L1=6.95 hemies; R=25,000 ohms; Q=102 (17) This shows that and. distributed capacitance is added by the outside tank and 11,500 ohms radiation resistance due to immersion of the back plate in castor oil. This compares to the theoretical value given by Equation 13 of The response curve measured in the experimental arrangement of Fig. 3 is shown in Fig. 12. The response is over a narrow band peaked at 58,500 cycles which is the resonant response of the composite unit. The calculated response of the unit working from the 100-ohm attenuator, compared to the response of a perfect transformer working between 100 ohms and 25,000 ohms is shown by the dotted line of Fig. 12. This is calculated from the equation:

Use of slotted plates and screens to cut down back radiation When oil surrounds both the crystal radiator and the quarter wave metal backing plate, the back radiation from the backing plate is down only 10 to 20 decibels depending on the type of backing resonator. This may not be sufficient to give the required front to back discrimination. It is the purpose of this invention to provide a simple method for cutting down the back radiation and increasing the front to back ratio.

The method depends on the fact that practically all the radiated back energy comes from the end of the backing resonator working into the radiation resistance and reactance of the medium. To cut down this radiation a slotted piece of steel is placed at a definite separation from the 8 end of the radiating back plate as shown in Fig. 5, which shows the experimental arrangement used to check the theory. The whole unit is immersed in castor oil which has a high viscosity, i. e., a value of 6.8 at 25 C. and considerably higher at lower temperatures.

It is well known that a single slot in a viscous medium will introduce a series resistance and reactance equal to z.= [%+1--p] (2 It is shown hereinafter that if the oil is compressed between a movable piston and an immovable wall that the reaction on the piston will be that of a shunt resistance and'mass having approximately the values per square centimeter 3 a .3 w a z.= 5;.- 1 -Z (21) where a. is the radius of the circular piston, p. the coeflicient of viscosity, p the density, and h the thickness of the liquid layer.

Applied to the device of Fig. 5, the equivalent circuit will be as shown in Fig. 6. The efiect of the slotted plate is to put a shunt impedance Z1 and a series impedance Z2 between the radiating piston and the radiation impedance. By adjusting these elements a considerable reduction of energy in the free liquid can be effected.

To test out the effect of a slotted plate in back, a plate 4 centimeters by 4 centimeters and ,4; inch or 0.318 centimeter thick, was provided having 19 slots 15 mils or 0.381 centimeter thick, with an average of 5.1 slots per centimeter. At 10,000 .cycles this unit will have a series impedance of .318 12X6.8 .g z Z| X27! X 10,000X 1] acoustic ohms per square centimeter. The radiation resistance of this unit. talnng it as being equivalent to a circular piston of the same area, for a frequency of 10,000 cycles will be Z1z=4.2 X 10 +i 8.24 X 10 (23) acoustic ohms per square centimeter. Taking a 20 mil separation, the shunt impedance Z1 will have a value of Z1=2.72 X 10 +i 6.5x 10 mechanical ohms per square centimeter.

With these values the output velocity is cut down in the ratio .742 which represents a loss of 2.6 decibels. Also the energy delivered to the output will be divided between the series resistance R2 and the radiation resistance Ra which represents a further loss of 5 decibels in the energy in 'the free liquid. Actually experiments show that the loss is larger than this which may be due to a cancellation of the energy radiated from the sides of the slotted plate compared to that radiated from the face.

This reduction has been tested experimentally by employing the arrangement shown in Fig. 5. The backing plate was placed in the lower chamber and the crystal mosaic in the upper chamber and both surrounded by oil. Previous experiments had shown that there was no appreciable transmission from the upper chamber to the lower. A 600 to 80,000-ohm transformer was inserted between the attenuator and the crystal, giving a higher input into the unit. The difference in reading between the oscillatorconnected directly to the amplifier, and the reading of the microphone, for back radiation from the backing resonator is shown by thedotted line of Fig. 13. The slotted plate was then placed at a definite separation from the backing resonator and the run repeated. For a six-mil separation. the difference curve is shown by the full line. This is from 10 to 20 decibels. below the radiation from the backing resonator alone, indicating a 15-decibel advantage in back radiation due to the slotted plate. The curve was repeated with l2-mi1 and 20-mil separations. The result of the'lZ-mil separation is shown by the dot-dash line. Although some points are lower, other points are higher so that the average attenuation is about the same as for the six-mil separation. The 20-mil separation, as shown by the dash, two dot curve, gave a definitely lower attenuation indicating that radiation was beginning to come from the slit-width.

Another use for a combination of this sort is to act as an attenuator in a complete radiator, as shown in Fig. 7. Here the backing resonators are completely surrounded by oil and back of these are a series of slotted plates or screens. The screens act as a series inductance and a resistance provided that they have enough mass (actual plug mass Of liquid that they drag along) to remain stationary. It has been found that most screens have enough eflective mass to make them act as series resistances and masses down to a frequency of 10,000 cycles or less. When the lateral dimension is very. wide and the spacing between screens is narrow, Equation 42 shows that the shunt impedance between screens reduces to where l is the thickness of the screen, It that of the gap between screens and A the eflective portion of a screen one centimeter square that is open. It is desirable to keep this impedance low so that the characteristic impedance of the screen system will not difier greatly from the impedance of the liquid. This will best be satisfied by a screen type structure in which the effective area A is made .5 or larger and the separations between screens in the order of .1 to .2 inch, resulting in an impedance nearly the same as the liquid. For example, for a 100 mesh to the inch screen made from four-mil wires and separated from the next screen by 0.1 inch, the impedance will be The extra resistance R put in by the screen will add a resistance R/2 to this characteristic impedance if we have a large number of screens together. It will also cause an attenuation of for every additional screen added to the assembly.

To test out this idea six layers of screen having 100, tour-mil wires to the inch were put together with a spacing of 100.mils and the attenuation of the combination measured. This gave quite uniformly over the frequency range down to 15 kilocycles an attenuation of three decibels. This indicates a value of R equal to .115 Zo=1.73X ohms per screen (29) which is close to what one would calculate from Equation 20. Next, six more layers were added and the attenuation measured again. Although the standing waves in the system lined with metal wool do not allow a very exact comparison at any one frequency, the average attenuation of the twelve screens was 6 decibels. Twenty layers of screening of this type back of the back resonators of Fig. 7 would introduce a loss of around 10 decibels. This would require a thickness of 2 inches or 50 centimeters. 7

Another use to which asystem of screens of this sort could be put is in providing a better absorbing layer around the edges of the measuring tank. The wool used in the measurements here introduce standing waves which may cause irregularities up to 5 decibels. By using a system of screens around the side and ends a considerably better absorption should be obtained.

There are two ways in which this could be accomplished. In the firstmethod screens are added on the outside till the series resistance of the screens is equal to the value of 22 for the liquid. This requires about five layers'of the screen measured above. The screen is then surrounded by a layer of material such as a thin layer of rubber which has a low acoustic imped. once. The termination will then be practically the resistance of the screening which should to match closely the radiation resistance of the medium.

The other method would be to pile on twenty or more layers of screening which would introduced loss of 10 decibels and prevent the impedance beyond the screening from aflecting the impedance at the screen surface. This would have an image impedance which would not differ much from that of the liquid, as shown by Equation 2'7. For a resistance termination not greater than (1.1.2) the radiation impedance v, it can be hown that standing waves will not affect the measurements by more than :1 decibel which is considerably better than has been obtained with a wool lining.

Impedance of a liquid layer The case of interest here i that shown in Fig. 9, where we have a piston compressing a liquid against an immovable wall. This case has been considered by Crandall (see Theory of vibrating systems and-sound, page 28, by Irving B. Crandall, published 1926 by D. Van Nostrand' Company), but Crandall neglects the mass of any contained layer and considers only the resistance. For a relatively wide separation such asconsidered here the mass reaction may be 1argei- -than the resistance reaction and hence cannot be neglected. The present embodiment derives the impedance of such a viscous layer when account:

11 12 Equation 20. Applied to the circular ring this ,2 Jog") becomes 7 *m (40) 12,: .Q I 1 To determine the impedance exerted on the vi- (2mflodr': ll 5 277% (30) 5 brating piston we integrate p over the surface since (zrrfilt) is the volume velocity, and 21:11:: and obtain the average value- This Wm be the cross-sectional area of the slit. This equa: 0 J00") tion which relates the radial particle velocity 1' pen SI w 21]; r[1 ]rdr= to the pressure reduces to +j%wp)+= 1,0, 1 kaJ (ka) (41) which give one of the relations of interest. a; g g impedance Per square centimeter v The other is given by the equation of continuen 8 ity. If we consider the elementary ring, this can Z: M v 2.),(ka) (42) be written in the form he: kaJ Gca) 1 g Q: If we expand the Bessels functions in power 1' br I bt (32) series, this impedance becomes where E is the velocityof the piston, and p the v= kw ka 11 3 excess density. This is related to p the excess M "'+7;+

pressure by the relation (43) (33) Inserting the value of 76 from (37) and collecting terms, the resistance and reactance terms so that the equation of continuity can be written become I =22? .22 222211 5. 21.) 2 I,'[H5 w 'soo v a wplf Q2 If we set p=0, i. e. neglect the mass, the re- 1 o +2i (34) actance becomes negative and the solution given 75 l, o" 35 by Crandall results. However, for the present Purpose this cannot be done. If the diameter is Inserting Equation 31 in 34 the to solve less than a half wave-length, and the spacing becomes la is in the order of 20 mils, only the first two 1 a 5p 12" terms are of importance and these are given in (r- +.7 wp]- 40 Equation 2.

What is claimed is:

2 213 (35) 1. In combination, a crystal radiator attached P" 5 at to a metal backing resonator and an acoustic 1 h manic m no resistance spaced beyond said resonator for abor for simp e a o n 45 sorbing radiations therefrom. 91 1 a 12a 2. In combination, a crystal radiator attached 23! ..E as r or l, v 5 v p l, l? 5" to a metal backing resonator, a housing for said radiator and its said backing resonator, a vis- (36) cous liquid medium in said housing surrounding If We denote by k: said radiator and its said backing resonator, and

a 6 a 12140: means for attenuating radiations from said res- 5 W 7: (37) onator to reduce the amount of energy transmitagsomuon of (37) is ted therefrom through said medium to said hous- 55 3: In combination, a crystal radiator attached P= n( ".71; (38) to a backing resonator, whereby mechanical movement of said combination under electrical It is-usual1y assumed that p=0 wh the stimulus and under ideal conditions of resonance radius of the piston since the liquid can expand is confined to a pistomnke movement of the free at the edge. Actually we should assume a radiafac of SM d r c tion resistance and mass termination at the edge ones of g ggi g g figgg g zgzfifiz gg put in by a radially vibrating ring representing a like pistomnke movement of the o pposite and the in and out motion the layer nquid- This free face of said resonator, an acoustic resistance impedance Wm be small and can usually be contiguous to said moving face of said resonator neglected if the thicknes la is small. The energy f at ti o communicated to the medium through this source, the g ggg g ggg gi f gf waves set up m however, will increase if It increases and sets a 4. In combination, a crystal radiator attached limit the gap that can profitably be used as a to a backing resonator, whereby mechanical shunt element movement of said combination under electrical a set when the constant A stimulus and under ideal conditions of resonance g face of said crystal and under practical condi- M tions of resonance is additionally manifested in a v like piston-like movement of the opposite and and the pressure at any point 1' will be given by!!! free face of said resonator, a viscous liquid is confined to a piston-like movement of the free fi,d15,83$ i R medium surrounding said combination of crystal movement of said combination under electrical and resonator, and an acoustic resistance constimulus and under ideal conditions of resonance tisuous to and in the path of vibrations set up in is confined to a piston-like movement of the free said medium by the said movement of said resface of said crystal and under practical condionator. 5 tions of resonance is additionally manifested in 5. In combination, a crystal radiator attached a like piston-like movement of the opposite and to a backing resonator, whereby mechanical free face of said resonator, a viscous liquid movement of said combination under electrical mediuin surrounding said combination of crystal stimulus and under ideal conditions 01' resonance and resonator, and an acoustic resistance conls confined to a piston-like movement of the tree 10 tiguous to and in the path of vibrations set up face of said crystal and under practical condiin said medium by the said movement of said tions of resonance is additionally manifested in a r so at r. d a oustic r sis an e comprising a like piston-like movement of the opposite and screen-like element.

free face of said resonator, viscou liquid 8. In combination, a. crystal radiator attached medium surrounding said combination of crystal 15 to a backing resonator, wh r y m anical and resonator, and an acoustic resistance conmovement of said combination under electrical tiguous to and in the path of vibrations set, up in stimulus and under ideal conditions of resonance said medium by the said movement of said resis confined to a piston-like movement of the free onator, said acoustic resistance comprising a thin face of said crystal and under practical condisheet-llke element having a plurality of spaced 2o tions of resonance is additionally manifested in openings therein. A, a like piston-like movement of the opposite and 6. In combination, a crystal radiator attached f f f S i resonator, a viscous liq i to a backing resonator, whereby mechanical medium ing s cflmbinatifln 1 cry l movement of said combination under electrical and. resonator, and an acoustic resistance constimulus and under ideal conditions of resonance 25 tis ou t0 an in the path of vibrations set up is confined to a piston-like movement of the free in said medium by the said movement of said face of said crystal and under practical condiresonator, said acoustic resistance comprising a tions of resonance is additionally manifested in a p ur y f layers of screen. like piston-like movement of the opposite and free face of said resonator, a viscous liquid m WARREN P. MASON. medium surrounding said combination of crystal and resonator and an acoustic resistance con- REFERENCES CITED tiguous to and in the path of vibrations set up in The following references are of record in the said medium by the said movement of said resm of thi at nt; 21:32:31,532 acoustic resistance comprising a UNITED STATES PATENTS 7. In combination, a crystal radiator attached Number a Date to a backing resonator, whereby mechanical 2,248,870 Langevin July 8, 1941 

